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* To whom correspondence should be addressed. E-mail: John.Huguenard{at}stanford.edu.
AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptors (AMPAR) mediate the majority of fast excitation in the CNS. Receptors lacking GluR2 exhibit inward rectification and paired-pulse facilitation (PPF) due to polyamine (PA)-dependent block and unblock, respectively. In the present study, we tested whether rectification and PPF in immature, but not mature, pyramidal neurons depend not only on the absence of functional GluR2 but also on the level of endogenous PAs. Whole cell recordings were obtained from layer V pyramidal neurons of P12-14 or P16-20 rats in the presence or absence of spermine in the pipette (50 µM). Isolated minimal excitatory synaptic responses were obtained and paired (20 Hz) stimuli were used to investigate the rectification index (RI) and paired-pulse ratio (PPR). Spermine and its synthetic enzyme, ornithine decarboxylase (ODC) expressions were examined using immunostaining and western blot, respectively. At the immature (< P15) stage inclusion of intracellular spermine increased RI and PPF for evoked EPSCs but had little or no effect on either measure in more mature (P16-20) pyramidal neurons. Depletion of PAs reduced rectification suggesting that endogenous PAs play a critical role in functional regulation of AMPARs. Spermine immunoreactivity and ODC expression in immature rat neocortex (< P15) were greater than more mature tissue by approximately 20% and 60%, respectively. These results provide further support for the idea that excitatory synapses on immature neocortical pyramidal neurons ubiquitously contain AMPA receptors lacking the GluR2 subunit and that the level of endogenous PAs plays an important role in modulating AMPAR-dependent neurotransmission.
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